A basic heat exchanger assembly, such as a condenser or evaporator for use in a motor vehicle or for use in residential/commercial applications including heat pumps, typically includes an inlet header manifold, an outlet header manifold, a plurality of refrigerant tubes hydraulically connecting the header manifolds for a two phase refrigerant flow therebetween, and external fins disposed between adjacent refrigerant tubes for increased heat transfer efficiency. The core assembly, also known as the center assembly, of the heat exchanger assembly is defined by the assembly of refrigerant tubes and corrugated fins interconnecting the refrigerant tubes. The header manifolds, tubes, and fins are typically assembled into a unitary structure and then brazed to form the unitary heat exchanger assembly.
Aluminum heat exchanger assemblies are desirable for their lightweight, ease of manufacturability, and heat transfer efficiencies. The refrigerant tubes are known to be manufactured by extruded aluminum alloys and are controlled atmosphere brazed (CAB) using a potassium fluoroaluminate flux. The CAB flux is applied to the heat exchanger assembly using an electrostatic fluxer or by wet slurry prior to the brazing of the heat exchanger assembly. Before brazing, the aluminum refrigerant tubes are processed with a pre-applied coating of zinc thermal spray at a weight of 8 to 12 g/m2. The Zn is diffused into the refrigerant tube aluminum substrate during brazing, thereby creating a sacrificial corrosion layer consisting of a gradient of Zn. Post braze, the Zn beneath the surface is approximately 4 to 7 wt % and diffused to a depth of approximately 100 microns, depending on the braze profile. The corrugated fins are typically formed of double sided clad aluminum alloy sheet. The header manifolds are typically manufactured of a single side clad aluminum sheet welded into a tube having corresponding slots for the insertion of the refrigerant tube ends.
This material system is sufficient for most automotive and non-automotive applications in geographic locations with nominal to low environmental pollutants that may corrode the aluminum substrate of the heat exchanger assembly. However, from a global perspective, there are regions of the world that have higher levels of environmental pollutants that may accelerate the corrosion of the aluminum heat exchanger assembly. Environmental pollutants can be acid rain, road de-icing salts, air pollutants, which includes diesel exhaust condensate, fertilizers, alkaline compounds and acetic and formic acids from construction, and seacoast chlorides. An aluminum heat exchanger assembly functioning as an evaporator sees corrosive condensates with pH values that can be as low as 4 from acid rain. The condensate accumulating on the heat exchanger assembly may have a high degree of conductivity caused by industrial pollution, in which the high conductivity increases the corrosive effect. There is a continual need to reduce the corrosion rates of aluminum heat exchangers to prolong service life in corrosive environments.